Blend of polytetrafluoroethylene, glass and polyphenylene sulfide fibers and filter felt made from same

ABSTRACT

An improved intimate cardable fiber blend containing polytetrafluoroethylene fibers, dual glass fibers and polyphenylene sulfide fibers and a filtration felt including a needled batt of the intimate cardable fiber blend made by mechanically blending the polyphenylene sulfide fibers and glass fibers with polytetrafluoroethylene fibers, carding the blend to form a nonwoven batt and needling the batt to provide a felt.

FIELD OF THE INVENTION

The present invention relates to an intimate fiber blend and more particularly to an intimate fiber blend including polytetrafluoroethylene, glass and polyphenylene sulfide fibers and a filter felt made therefrom.

BACKGROUND OF THE INVENTION

Use of bag filters as filters for collecting the dust emitted, for example, from refuse incinerators, coal boilers and metal melting furnaces, is well-known. In these applications, bag filters are required to exhibit heat resistance, since the exhaust gas temperatures are in a high temperature range of 150° C. to 250° C., depending on application. The conventional filter media used at such high temperatures are made of felt produced by laminating a ground fabric and a web using polyphenylene sulfide fibers, metaaramid fibers, polyimide fibers, fluorine fibers or glass fibers, etc., and entangling the fibers using a needle punch or jet water stream, etc.

Filters containing polytetrafluoroethylene fibers are advantageous because they have outstanding resistance to high temperatures, chemical attack and abrasion. However, commercially available filters containing these fibers are expensive and often permit passage of more particulate matter (PM) than is desirable under today's increasingly rigorous environmental standards. Polyphenylene sulfide fibers also have excellent properties such as heat resistance, barrier properties and chemicals resistance, but like filters containing polytetrafluoroethylene filter felt, polyphenylene sulfide filter felts permit passage of more particulate matter than is desired, though typically not as much as polytetrafluoroethylene felts. Such filter media exhibit another shortcoming. Polyphenylene sulfide fiber filters have poor burn-through resistance when contacted by sparks, which are often present in the effluent of incinerators, coal boilers and metal melting furnaces. The holes that result from spark burn-through can result in decreased filtration performance of such filters.

An alternative to filter felts for use in bag filters are membrane-type filters. Generally, membrane filters are expected to have a greater filter efficiency than that of filter felts. In part, that is because membrane filters have pores of a controlled and predetermined size through which particulate laden air can pass. The pores are small enough to capture particulate matter that conventional filter felts cannot. However, the small pore size of today's membrane filters often causes them to produce an undesirably high pressure drop across the filters which relates to decreased air permeability and ultimately decreased filtration performance. This is particularly true for membrane filters designed to capture fine particles, i.e., particles less than 2.5 micrometers in diameter, which are believed to pose the greatest health risks. Consequently, the overall filtration performance of such membrane filters is offset by poor air permeability.

Another shortcoming of membrane filters is that they are often too fragile to be implemented in a certain application. For example, the filtration of hot gases produced from the manufacture of asphalt regularly occurs in an environment that can damage membrane filters, which results in the decrease service life of such filters. This happens in part because of the nature of the equipment used in the asphalt industry and its inadequate upkeep. In addition, membrane filters are expensive to use compared, for example, to polyphenylene sulfide fiber filter felts.

The benefit of the filter felt of the present invention is its unexpected ability to meet or exceed the filtration efficiency of membrane filters while providing a lower pressure drop across the filter. This allows for a more cost effective filtration design for a bag house. In addition, the present invention is intended to solve the above problems by providing a filter felt having good heat and burn-though resistance properties and improved strength as a filter medium when used at high temperatures of 150° C. to 250° C. in refuse incinerators, coal boilers, metal melting furnaces, etc. Further, the filter felt of the present invention will likely exhibit a longer service life than conventional 100% polyphenylene sulfide filter felts when used extensively at high temperatures of 190° C. to 205° C.

SUMMARY OF THE INVENTION

This invention provides an intimate cardable fiber blend containing 5% to 15% of 2 to 25 denier per filament polytetrafluoroethylene fibers, 25% to 55% of 0.1 to 1 denier per filament glass fibers and 1 to 10 denier per filament polyphenylene sulfide fibers. The invention also provides an improved filter felt comprising a needled batt of the intimate cardable fiber blend. Preferably, the blend includes 30% to 50% by weight glass fibers having lengths ranging between 1.5 inches and 4 inches and a more preferably lengths of about 1.5 inches with an average diameter of 6 microns. Further, it is preferred that the fiber blend includes approximately 10% by weight of 3 to 7 denier per filament polytetrafluoroethylene fibers having lengths ranging between 2.0 inches and 4.5 inches and more preferably lengths of about 3 inches and a denier per filament of 3.5 to 6.7. In addition, the polyphenylene sulfide fibers preferably represent from 40% to 60% by weight of the blend and are 2 to 7 denier per filament polyphenylene sulfide fibers having lengths ranging between 2.0 inches and 4.5 inches and more preferably lengths of about 3 inches and a denier per filament of 2.5 to 3.

This invention also provides a process for preparing the filter felt by (1) mechanically blending the polyphenylene sulfide fibers and glass fibers with polytetrafluoroethylene fibers, (2) further blending the fibers in a carding machine, forming a nonwoven batt, if necessary by crosslapping, (3) combining layers of the batt to form a layered batt of a desired thickness, (4) needling the batt to provide a felt and, (5) optionally, heat setting the felt by heating on a tenter frame. Preferably the filter felt contains a supporting scrim which most preferably is a woven fabric of polyphenylene sulfide fibers, although other synthetic fibers are contemplated such a polytetrafluoroethylene fibers. The mechanical blending can be accomplished by any means known in the art, for example, by hand or in a picker or by creeling the fibers in one creel and then forming a tow which is crimped and cut to the desired cut length or in a mechanical blending device like a fiber opener.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

This invention relates to an intimate blend of fibers and a filter felt made therefrom, the blend including glass fibers, polytetrafluoroethylene fibers and polyphenylene sulfide fibers. The combination of the fibers unexpectedly results in a filter felt exhibiting improved resistance to burn-through by sparks, hot embers and the like, improved filter efficiency and improved strength and degradation characteristics.

Useful glass fibers are typical continuous or spun glass fiber available commercially from Owens-Coming and AGY. The glass fibers can be cut to desired staple length on a Lummus cutter. For ease of processing, crimped glass fibers or dual glass fibers can be used. The term “dual glass fiber” as used herein means a glass fiber made from two or more glass compositions having different coefficients of expansion. Dual glass fibers may also be known as irregularly-shaped glass fibers or bi-glass fibers. These glass fibers are not straight, but instead curl after spinning producing a natural, random twist. Dual glass fiber is sold by Owens-Coming under the MIRAFLEX name. The preferred glass fiber is “DE” type glass fiber.

The term “fluoropolymer fiber” as used herein means a fiber prepared from polymers such as polytetrafluoroethylene, and polymers generally known as fluorinated olefinic polymers, for example, copolymers of tetrafluoroethylene and hexafluoropropene, copolymers of tetrafluoroethylene and perfluoroalkyl-vinyl esters such as perfluoropropyl-vinyl ether and perfluoroethyl-vinyl ether, fluorinated olefinic terpolymers including those of the above-listed monomers and other tetrafluoroethylene based copolymers. For the purposes of this invention, the preferred fluoropolymer fiber is polytetrafluoroethylene fiber.

The fluoropolymer fiber can be spun by a variety of means, depending on the exact fluoropolymer composition desired. Thus, the fibers can be spun by dispersion spinning; that is, a dispersion of insoluble fluoropolymer particles is mixed with a solution of a soluble matrix polymer and this mixture is then coagulated into filaments by extruding the mixture into a coagulation solution in which the matrix polymer becomes insoluble. The insoluble matrix material may later be sintered and removed if desired. One method which is commonly used to spin polytetrafluoroethylene and related polymers includes spinning the polymer from a mixture of an aqueous dispersion of the polymer particles and viscose, where cellulose xanthate is the soluble form of the matrix polymer, as taught for example in U.S. Pat. Nos. 3,655,853; 3,114,672 and 2,772,444. Preferably, the fluoropolymer fiber of the present invention is prepared using a more environmentally friendly method than those methods utilizing viscose. One such method is described in U.S. Pat. Nos. 5,820,984; 5,762,846, and 5,723,081. In general, this method employs a cellulosic ether polymer such as methylcellulose, hydroxyethylcellulose, methylhydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose, ethylcellulose or carboxymethylcellulose as the soluble matrix polymer, in place of viscose. Alternatively, if melt viscosities are amenable, filament may also be spun directly from a melt. Fibers may also be produced by mixing fine powdered fluoropolymer with an extrusion aid, forming this mixture into a billet and extruding the mixture through a die to produce fibers which may have either expanded or un-expanded structures. For the purposes of this invention, the preferred method of making the fluoropolymer fiber is by dispersion spinning where the matrix polymer is a cellulosic ether polymer.

The fluoropolymer fiber can be made into the desired staple length using any number of means known in the art. Preferably, the fluoropolymer fiber is cut into staple by a Lummus cutter. Polytetrafluoroethylene staple fiber is sold by Toray Fluorofibers (America), Inc.

The polyphenylene sulfide fibers used in the present invention are known to be excellent in heat resistance, chemical resistance and hydrolysis resistance. Preferably, the fibers contain 90% or more of fibers made of a polymer containing the phenylene sulfide structure —(C₆H₄—S)_(n)— (n is an integer of 1 or more) as a component of the fibers. The polyphenylene sulfide fibers can be cut into the desired staple length by any number of means known in the art, including by using a Lummus cutter. Methods for preparing polyphenylene sulfide fibers are described in U.S. Pat. Nos. 3,898,204 and 3,912,695. Polyphenylene sulfide staple fibers are sold by Toray Industries, Inc.

The filter felt of this invention can be prepared by any means known in the art. For example, the felt can be prepared by (1) making a fiber blend containing about 10% polytetrafluoroethylene fibers, 40% to 60% polyphenylene sulfide fibers and 30% to 50% glass fibers in a picker, (2) passing the blend through a suitable carding machine to provide a web of an intimate blend of polytetrafluoroethylene, polyphenylene sulfide and glass fibers, (3) cross-lapping the carded web from the carding machine and combining the resulting batt into a layered batt, if necessary, to provide the desired weight, preferably between 5 and 25 ounces per square yard and most preferably about 16 ounces per square yard, (4) lightly needling the layered batt on one or both sides using a needle loom, and (5) further needling the batt both sides of a woven polyphenylene sulfide scrim to produce a felt. The batts of blended fibers may also be prepared using an air-lay. If desired, the felt can be heat set by placing the uncompacted felt on a tenter frame and passing the felt through an oven.

A preferred embodiment of the present invention includes an intimate fiber blend and filter felt made therefrom comprising about one part polytetrafluoroethylene staple fibers, about 3 parts glass staple fibers and about 6 parts polyphenylene sulfide staple fibers wherein the polyphenylene sulfide staple fibers are present as 2.7 denier polyphenylene sulfide staple fibers and the polytetrafluoroethylene staple fibers are present as 6.7 denier polytetrafluoroethylene staple fibers and 3.5 denier polytetrafluoroethylene staple fibers.

A further preferred embodiment of the present invention includes an intimate fiber blend and filter felt made therefrom comprising about one part polytetrafluoroethylene staple fibers, about 4 parts glass staple fibers and about 5 parts polyphenylene sulfide staple fibers wherein the polyphenylene sulfide staple fibers are present as 2.7 denier polyphenylene sulfide staple fibers and the polytetrafluoroethylene staple fibers are present as 6.7 denier polytetrafluoroethylene staple fibers and 3.5 denier polytetrafluoroethylene staple fibers.

Another preferred embodiment of the present invention includes an intimate fiber blend and filter felt made therefrom comprising about one part polytetrafluoroethylene staple fibers, about 5 parts glass staple fibers and about 4 parts polyphenylene sulfide staple fibers wherein the polyphenylene sulfide staple fibers are present as 2.7 denier polyphenylene sulfide staple fibers and the polytetrafluoroethylene staple fibers are present as 6.7 denier polytetrafluoroethylene staple fibers and 3.5 denier polytetrafluoroethylene staple fibers.

The present invention will be explained further in detail by the following example.

EXAMPLE

A test filter felt according to the present invention was prepared by producing a first fiber blend containing 5% by weight of polytetrafluoroethylene fibers having an average length of 3 inches and a denier per filament of 3.5; 5% by weight of polytetrafluoroethylene fibers having an average length of 3 inches and a denier per filament of 6.7; 50% by weight of polyphenylene sulfide fibers having an average length of 3 inches and denier per filament of 2.7, and 40% by weight of DE fiber glass having an average length of 3 inches and average diameter of 6 microns. A second fiber blend was prepared containing 5% by weight of polytetrafluoroethylene fibers having an average length of 3 inches and a denier per filament of 3.5; 5% by weight of polytetrafluoroethylene fibers having an average length of 3 inches and a denier per filament of 6.7; 60% by weight of polyphenylene sulfide fibers having an average length of 3 inches and denier per filament of 2.7, and 30% by weight of DE fiber glass having an average length of 3 inches and average diameter of 6 microns.

Each of the first blend and the second blend was blended mechanically and further blended in a commercial carding machine to provide a first web and a second web, respectively, of an intimate blend of polytetrafluoroethylene, polyphenylene sulfide and glass fibers. Thereafter, each of the carded webs from the carding machine was cross-lapped to provide a pair of batts having the desired weights. To produce the test filter felt, the batt of the first blend was needled using a needle loom on one side of a woven polyphenylene sulfide scrim and the batt of the second blend was needled using the needle loom on the other side of the polyphenylene sulfide scrim.

TEST RESULTS AND MEASUREMENTS Basis Weight and Thickness

The test filter felt had an average weight of 16.4 ounces per square yard with a range of 14.8 to 18.2 ounces per yard and an average thickness of 0.082 inches with a range of 0.074 to 0.088 inches.

Air Permeability

The test filter felt exhibited an air permeability average of 35.1 cubic feet per minute with a range of 25.2 to 40.4 cubic feet per minute.

Mullen Burst Strength

The burst strength of the test filter felt was measured using the Mullen Burst Test. The Mullen Burst Test uses a circular material sample that has been clamped over a diaphragm and inflated with oil. Pressure is applied until the test fabric bursts. The pressure (in pounds per square inch) at which the fabric bursts is the bursting strength. The burst strength of the test filter felt as measured by a Mullen Burst Test averaged 268 pounds per square inch with a range of 252 to 298 pounds per square inch.

Ball Bearing Test

The test filter felt's resistance to melting was tested using a ball bearing test. The test includes suspending the test filter felt across one open horizontal frame and a control felt consisting of a 100% polyphenylene sulfide fiber felt having a weight per square yard substantially equal to the weight per square yard for the test filter felt across another open horizontal frame. One or two stainless steel ball bearings of ¼ to ½ inch are heated to about 343° C. in a Blue Max type oven or a muffle furnace. This is above the melting point of polyphenylene sulfide fiber. A high temperature pad is placed beneath each of the frames to receive the ball bearings if they penetrate the test filter felt or the control felt. The heat sink effect of the steel is high so residence time in the oven is required. A heated ball bearing is placed on each of the test filter felt and control felt, and the felts are observed. In the test, the heated ball bearing failed to penetrate the test filter felt. However, the control felt was melted and penetrated by the heated ball bearing.

Burn-Through Resistance

The test filter felt's resistance to burn-through from sparks, for example, as encountered by bag houses in coal fired boilers, asphalt plants and metal working facilities, was measured using a hot ember test. The potential for burn-through of the test filter felt was measured against a control filter felt consisting of 100% polyphenylene sulfide felt with scrim having an average weight of 16 ounces per square yard. The test consisted of contacting a red hot ember at the end of a wooden stick to the test filter felt and the control filter felt.

In the test, the control felt initially resisted burn-through when contacted by the red hot ember but eventually the glowing ember penetrated the control felt. The test felt resisted burn-through when contacted with red hot ember even when substantial pressure was exerted by the ember against the test filer felt. No holes were burned through the test filter felt.

Filter Performance

Testing of the resulting test filter felt was conducted using an ETS, Inc. Filtration Performance Test Apparatus to determine the filter sample's performance with respect to outlet particulate emissions (PM2.5), outlet particulate emissions (total mass), initial residual pressure drop, increase in residual pressure drop, average residual pressure drop, mass weight gain of the filter sample, average filtration cycle time and number of filtration cycles. Testing was conducted in accordance with ASTM Test Method D6830-02 and with the test specifications and conditions as detailed in the Generic Verification Protocol for Baghouse Filtration Products (BFP) developed by the Air Pollution Control Technology Verification Center (APCTVC) which is part of the U.S. EPA's Environmental Technology Verification (ETV) Program and is operated in partnership between RTI and EPA. The protocol was adapted from the German VDI Method 3926, and modified for ETV. One exception to the protocol specification was that the test program consisted on one run rather then three runs as specified in the protocol.

The test run consisted of three test phases. To simulate long term operation, the filter sample was first subjected to a conditioning period which consisted of 10,000 rapid pulse cleaning cycles under continuous dust loading. During this period, the time between cleaning cycles was maintained at three seconds. No filter performance parameters were measured during the conditioning period.

The conditioning period was immediately followed by a recovery period, which allowed the filter felt sample to recover from the rapid pulsing. The recovery period consisted of 30 normal filtration cycles under continuous dust loading. During a normal filtration cycle, the dust cake was allowed to form on the test filter felt until a differential pressure of 1,000 Pa (4.0 inch w.g.) was reached. At this point, the test filter felt was cleaned by a pulse of compressed air. Immediately after pulse cleaning the pressure fluctuated rapidly inside the test duct. Some of the released dust immediately re-deposited on the test filter felt. The pressure then stabilized and returned to normal. Thus, the residual pressure drop across the filter felt was measured three seconds after conclusion of the cleaning pulse. It was monitored and recorded continuously throughout the recovery and performance test period.

The performance test period immediately followed the recovery period for a cumulative total of 10,030 cycles after the test filter felt was installed in the test apparatus. The performance test period was six hours in duration and during this phase normal filtration cycles and constant dust loading were maintained and recorded. Outlet mass and PM 2.5 dust concentrations were measured using an inertial impactor located downstream of the test filter felt. The weight gain of each impactor stage substrate was measured to within 0.00001 grams.

Test conditions throughout the test were as follows: Test dust: Pural NF Alumina (1.5±1.0 micron mass mean diameter); Inlet dust feed rate: 100±20 grams/hr. (18.4±grams/scm); Filtration Velocity: 120±6 m/hr; Gas Temperature: 25° C.±2° C., and Pulse Cleaning Pressure: 75 psi.

A control filter felt manufactured by Southern Felt was also tested using the test method and specification described above. The control filter felt consisted of standard 100% polyphenylene sulfide filter felt having a weight ranging between 16 to 19 ounces per square yard.

The test results for the performance test phase are summarized in Table 1. TABLE 1 Parameter Standard PPS Test Felt PM2.5 Emissions (g/dscm) 0.0000815 0.0000249 Total Mass Emissions (g/dscm) 0.0000830 0.0000249 Initial Residual Pressure Drop (cm w.g.) 3.15 2.28 Residual Pressure Drop Increase (cm w.g.) 0.51 1.39 Average Residual Pressure Drop (cm w.g.) 3.43 3.03 Filter Sample Weight Gain (grams) 1.58 1.82 Average Filtration Cycle Time (seconds) 122 111 Number of Filtration Cycles (or Pulses) 176 195

Detailed information for the performance test phases is provided in Table II. TABLE II Parameter Standard PPS Test Felt VERIFICATION TEST RESULTS Mean Outlet Particle Conc. 0.0000815 0.0000249 PM 2.5 (g/dscm) Mean Outlet Particle Conc. 0.0000830 0.0000249 Total mass (g/dscm) Initial Residual Pressure 3.15 2.28 Drop (cm w.g.) Change in Residual Pressure 0.51 1.39 Drop (cm w.g.) Average Residual Pressure 3.43 3.03 Drop (cm w.g.) Mass Gain of Filter Sample (g) 1.58 1.82 Average Filtration Cycle Time (s) 122 111 Number of Pulses 176 195 RESIDUAL PRESSURE DROP At Start of: Conditioning Period (cm w.g.) 0.18 0.24 Recovery Period (cm w.g.) 3.06 2.23 Performance Test Period (cm w.g.) 3.15 2.28 Pulse Pressure (psi) 75 75 REMOVAL EFFICIENCY (%) Dust Conc (g/dscm) 18.10 16.80 PM 2.5* 99.99942 99.99981 Total Mass** 99.99954 99.99985 $*\frac{\left( {{Dust}\quad{Concentration}\quad*\quad 0.7735} \right) - {{PM}\quad 2.5\quad{Outlet}\quad{Concentration}}}{{Dust}\quad{Concentration}\quad*\quad 0.7735}*\quad 100$ ${**\frac{\left( {{{Dust}\quad{Concentration}} - {{Total}\quad{Mass}\quad{Outlet}\quad{Concentration}}} \right.}{{Dust}\quad{Concentration}}}*\quad 100$

Comparison to Membrane-Type Filter Media

Performance testing using the ETS, Inc. Filtration Performance Test Apparatus was conducted in accordance with ASTM Test Method D6830-02 on a control filter felt laminated with a Textratex® expanded polytetrafluoroethylene membrane style 8005 available from Donaldson Company, Inc. of Bloomington, Minn. The test specifications and conditions were as detailed in the Generic Verification Protocol for BFP developed by the APCTVC. The membrane exhibited PM 2.5 emissions of 0.00005 g/dscm and an initial pressure drop of 8.46 cm w.g. versus 0.0000249 g/dscm and 2.28 cm w.g., respectively, for the test filter felt. In addition, the membrane exhibited a filter sample weight gain of 0.16 grams versus 1.82 grams for the test filter felt.

As will be apparent to one skilled in the art, various modifications can be made within the scope of the aforesaid description. Such modifications being within the ability of one skilled in the art form a part of the present invention and are embraced by the claims below. 

1. An intimate blend of fibers comprising polyphenylene sulfide fibers, glass fibers and fluoropolymer fibers.
 2. The blend according to claim 1 including from about 20% to about 70% glass fibers.
 3. The blend according to claim 2 including from about 30% to about 50% glass fibers.
 4. The blend according to claim 3 wherein the glass fibers are dual glass fibers.
 5. The blend according to claim 3 wherein the glass fibers are between about 1.5 inches and 4 inches in length and have a diameter of about 6 microns.
 6. The blend according to claim 1 wherein the blend includes from about 40% to about 60% polyphenylene sulfide fibers.
 7. The blend according to claim 6 wherein the polyphenylene sulfide fibers are between about 2.0 inches and about 4.5 inches in length and have a denier per filament of between about 2 and about
 7. 8. The blend according to claim 1 including from about 5% to about 30% polytetrafluoroethylene fibers.
 9. The blend according to claim 8 including from about 7% to about 20% polytetrafluoroethylene fibers.
 10. The blend according to claim 1 including from about 40% to about 60% polyphenylene sulfide fibers, from about 30% to about 50% glass fibers and about 10% polytetrafluoroethylene fibers.
 11. The blend according to claim 1 wherein the polyphenylene fibers are present in an amount less than that of the glass fibers.
 12. The blend according to claim 1 wherein the ratio of glass fibers and polyphenylene fibers to fluoropolymer fibers is about 9 to
 1. 13. The blend according to claim 1 wherein the ratio of glass fibers and fluoropolymer fibers to polyphenylene fibers is from about 2 to 3 to about 3 to
 2. 14. The blend according to claim 1 wherein the ratio of fluoropolymer fibers and polyphenylene fibers to glass fibers is from about 1 to 1 to about 7 to
 3. 15. The blend according to claim 1 wherein for every one part fluoropolymer fibers there is between about 3 to about 5 parts glass fibers and between about 4 to about 6 parts polyphenylene sulfide fibers.
 16. The fiber blend according to claim 1 comprising about one part polytetrafluoroethylene staple fibers, about 3 to about 5 parts glass staple fibers and about 4 to about 6 parts polyphenylene sulfide staple fibers.
 17. A filter comprising a membrane-type filter laminated to a filter felt prepared from the intimate fiber blend of claim
 1. 18. The filter according to claim 17 wherein the membrane-type filter is a polytetrafluoroethylene membrane.
 19. An improved filter felt comprising a needled batt of the intimate fiber blend of claim
 1. 20. The felt according to claim 19 having a weight ranging between about 14.8 ounces per square yard to about 18.2 ounces per square yard.
 21. The felt according to claim 19 having a thickness ranging between about 0.074 inches to about 0.088 inches.
 22. The felt according to claim 19 wherein the supporting scrim is made of at least one of polyphenylene sulfide fibers and polytetrafluoroethylene fibers.
 23. The felt according to claim 19 wherein the felt exhibits a PM 2.5 emissions result of about 0.0000249 grams per dry standard cubic meter according to ASTM Test Method D6830-02.
 24. The felt according to claim 19 wherein the felt exhibits a total mass emissions result of about 0.0000249 grams per dry standard cubic meter according to ASTM Test Method D6830-02.
 25. The felt according to claim 19 wherein the felt exhibits an initial residual pressure drop of about 2.28 centimeters of water gauge according to ASTM Test Method D6830-02.
 26. The felt according to claim 19 wherein the felt exhibits a residual pressure drop increase of about 1.39 centimeters of water gauge according to ASTM Test Method D6830-02.
 27. The felt according to claim 19 wherein the felt exhibits an average residual pressure drop of about 3.03 centimeters of water gauge according to ASTM Test Method D6830-02.
 28. The felt according to claim 19 wherein the felt exhibits a filter sample weight gain of about 1.82 grams according to ASTM Test Method D6830-02.
 29. The felt according to claim 19 wherein the felt exhibits a PM 2.5 removal efficiency of about 99.99981% according to ASTM Test Method D6830-02.
 30. The felt according to claim 19 wherein the felt exhibits a total mass removal efficiency of about 99.99985% according to ASTM Test Method D6830-02.
 31. The felt according to claim 19 wherein the felt exhibits a permeability of about 25.2 cubic feet per minute to about 40.4 cubic feet per minute.
 32. The felt according to claim 19 wherein the felt exhibits a Mullen Burst strength average of about 252 pounds per square inch to about 298 pounds per square inch.
 33. The felt according to claim 19 wherein the felt exhibits, according to ASTM Test Method D6830-02, less PM 2.5 emissions than that of a 100% polyphenylene sulfide fiber filter felt material having a weight per square yard substantially equal to the weight per square yard of the felt.
 34. The felt according to claim 19 wherein the felt exhibits a Mullen Burst strength average greater than that of a 100% polyphenylene sulfide fiber filter felt material having a weight per square yard substantially equal to the weight per square yard of the felt.
 35. The felt according to claim 19 wherein the felt exhibits a greater resistance to burn-through from a spark than that of a 100% polyphenylene sulfide fiber filter felt material having a weight per square yard substantially equal to the weight per square yard of the felt.
 36. The felt according to claim 19 wherein the felt exhibits a burn-through temperature greater than that of a 100% polyphenylene sulfide fiber filter felt material having a weight per square yard substantially equal to the weight per square yard of the felt.
 37. A process for preparing the filter felt of claim 19 comprising, mechanically blending about 3 to about 5 parts glass staple fibers and about 4 to about 6 parts polyphenylene sulfide staple fibers to every one part polytetrafluoroethylene staple fibers, carding and crosslapping the fibers with a carding machine, and needling the batt onto one or both sides a scrim of polyphenylene sulfide fibers.
 38. The felt according to claim 19 wherein the felt resists melt-through by one or more stainless steel ball bearings of ¼ to ½ inch, heated to about 343° C., when the heated one or more ball bearings are placed on the filter felt while the filter is stretched horizontally across a frame.
 39. An intimate fiber blend comprising about one part polytetrafluoroethylene staple fibers, about 3 parts glass staple fibers and about 6 parts polyphenylene sulfide staple fibers wherein the polyphenylene sulfide staple fibers are present as 2.7 denier polyphenylene sulfide staple fibers and the polytetrafluoroethylene staple fibers have a denier per filament of between 3.5 and 6.7.
 40. An intimate fiber blend comprising about one part polytetrafluoroethylene staple fibers, about 4 parts glass staple fibers and about 5 parts polyphenylene sulfide staple fibers wherein the polyphenylene sulfide staple fibers are present as 2.7 denier polyphenylene sulfide staple fibers and the polytetrafluoroethylene staple fibers have a denier per filament of between 3.5 and 6.7.
 41. An intimate fiber blend comprising about one part polytetrafluoroethylene staple fibers, about 5 parts glass staple fibers and about 4 parts polyphenylene sulfide staple fibers wherein the polyphenylene sulfide staple fibers are present as 2.7 denier polyphenylene sulfide staple fibers and the polytetrafluoroethylene staple fibers have a denier per filament of between 3.5 and 6.7. 